The skeleton is the body part that forms the supporting structure of an organism. There are several different skeletal types: the exoskeleton which is the stable outer shell of an organism the endoskeleton which forms the support structure inside the body the hydroskeleton and the cytoskeleton.
Exoskeletons are external and are found in many invertebrates; they enclose and protect the soft tissues and organs of the body. Some kinds of exoskeletons undergo periodic moulting as the animal grows as is the case in many arthropods including insects and crustaceans.
The exoskeleton of insects is not only a form of protection but also serves as a surface for muscle attachment as a watertight protection against drying, and as a sense organ to interact with the environment. The shell of mollusks also performs all of the same functions except that in most cases it does not contain sense organs.
An external skeleton can be quite heavy in relation to the overall mass of an animal so on land organisms that have an exoskeleton are mostly relatively small. Somewhat larger aquatic animals can support an exoskeleton because weight is less of a consideration underwater. The southern giant clam a species of extremely large saltwater clam in the Pacific Ocean has a shell that is massive in both size and weight. Syrinx aruanus is a species of sea snail with a very large shell.
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